The present invention relates to orthopedic implants and more specifically to consolidated ceramic reinforced polymers useful in orthopedic implant applications.
For at least three decades, ultra high molecular weight polyethylene (UHMWPE) has dominated the joint replacement arena as the material of choice in joint prostheses, i.e. primarily hip and knee replacements.
The principle limitation controlling the longevity of hip prostheses is the body""s reaction to UHMWPE debris generated during interaction with the articulating femoral ball. The UHMWPE debris has been linked to bone resorption (osteolysis) which leads to loosening and premature failure of the implant.
Failure of total knee replacements is largely associated with high stress fatigue loads that result in pitting, crack propagation and delamination of UHMWPE in such applications. For example, normal activities such as walking and climbing stairs can impose forces between the tibia and femur that are approximately four to five times ones body weight. Recent studies have shown that out of 937 tibial bi-compartmental and uni-compartmental bearings, 50% had indications of delaminations and cracking. In fact, total joint implants rarely last over ten years today. The rapidly growing aging population and the increasing use of implants in younger people provides a strong mandate for longevity improvements in total joint arthroplasties. Thus, numerous approaches to improving the useful life of such devices have been and continue to be proposed.
Among the proposed improvements are; advanced crosslinking methods, carbon fiber reinforcement, and the use of a variety of counterface materials including CoCr, Ti-6Al-4V, and ceramics. Quite recently, surface engineering of metal counterfaces, e.g. coating, implantation, oxidation and diffusion has also been evaluated for reducing the wear rate of the preferred UHMWPE materials.
Currently, most standard polyethylene components are irradiated with 5 Mrad of gamma radiation in a nitrogen atmosphere and the heated to 155xc2x0 C. for 24 hours to remove free radicals before being machined into components. Considerable research has been and continues to be expended to reduce wear rates and creep in UHMWPE since none of the foregoing methods has supplied a satisfactory solution.
A method that uses high pressure to produce a more highly crystallized polyethylene (PE) (80% compared to 55% for normal irradiated PE) produced a material that exhibited higher yield stress, modulus and resistance to creep and crack growth, but neither laboratory wear tests nor clinical trials have demonstrated improvement in wear resistance.
Numerous efforts have attempted to improve the performance of UHMWPE by increasing the cross-linking to improve its wear resistance. These efforts have included the application of higher doses of gamma radiation, electron beam radiation, and heating in combination with irradiation and chemical peroxide treatments to increase the degree of cross-linking.
The mechanical properties of enhanced cross-linked PEs show a decrease in yield stress, elastic modulus, tensile strength, creep resistance, toughness and elongation to failure accompanied by an improvement in wear resistance. Since strength and creep resistance are crucial to extending the life of prostheses it is essential that these properties not be negatively impacted as other properties such as wear resistance are enhanced.
Alternative materials have also been used to approach the solution to the prosthesis wear problem. Several different polymeric materials have, for example, been evaluated over the years. Polytetrafluoroethylene, various polyesters and polyacetyls have been evaluated without finding a successful replacement for UHMWPE.
Chopped carbon fiber reinforced PE while demonstrating promising results in bench scale screening and simulator tests failed in clinical trials. Chopped carbon fiber reinforced epoxy-based cups articulating against alumina heads have demonstrated five times lower wear rates than UHMWPE cups in early clinical trials.
In summary, although a large number of solutions have been proposed to provide improved hip and knee replacement materials, UHMWPE remains the current, if not entirely satisfactory, material of choice in such applications and the provision of an improved material or combination of materials which would provide improved wear resistance and strength for such uses remains an elusive objective.
It is therefore an object of the present invention to provide an improved material for use in orthopedic implants.
It is another object of the present invention to provide such an improved material that exhibits enhanced wear resistance and strength over existing and currently used UHMWPE materials.
It is yet a further object of the present invention to provide improved orthopedic implants that exhibit useful lives beyond those implants currently in use today.